1. Field of the Invention
The present invention relates to polyethylene terephthalate (PET), and in particular relates to a nucleating agent and an injection molding method for PET.
2. Description of the Related Art
Contemporary material science has many development branch including metals, alloys, ceramics, and polymers. Polymers are most complex thereof because many properties such as viscosity, crystallinity, and melting point are influenced by types of monomers, degree of polymerization, arrangements of the repeating unit, and crosslinking between the polymers. These properties determine the applications and types of molding methods of the polymers. Conventional methods of regulating these properties include altering the monomer to initiator ratio, polymerizing temperature and pressure, and adding nucleating agents to accelerate crystallization of the polymers.
Polyethylene terephthalate (PET) is inexpensive, however; it cannot be used in injection molding because of its slow crystallization rate at high temperatures. On the other hand, PET shaped articles formed at low temperature (60 to 70° C.) are susceptible to degradation. In U.S. Pat. No. 4,272,475, Chi discloses a method for controlling the cylinder temperature, the nozzle temperature, the injection temperature, and the shot time for PET injection articles, but the molecular weight of the articles is too low to be used in engineering-grade plastics. The described problems result from low crystallization rates of high-molecular-weight PET articles at high temperatures. As high molding temperature is required for PET melt to crystallize, an oil bath system is necessary for PET molding. Compared with the water bath system used for polybutylene terephthalate (PBT), the oil bath system for PET is more costly and dangerous.
For increasing the crystallization rate of PET, U.S. Pat. No. 4,451,606 discloses an organic nucleating agent, which is a copolymer of poly(alkylene terephthalate) and a sulfonate salt of an aromatic dicarboxylic acid. This organic nucleating agent can be miscible with PET and cause uniform nucleation. The organic nucleation agent, however, may lose its function at high temperature (about 280-300° C.) due to disproportion or degradation.
To avoid the problems of the organic nucleating agents, inorganic nucleating agents to improve PET crystallization rate have been adopted. As disclosed in U.S. Pat. No. 5,886,088, suitable inorganic nucleating agents include talc, mica, wollastonite, clay, kaolin, diatomaceous earth, bentonite, montmorillonite, hydrotalcite, calcium carbonate, titanium oxide, potassium titanate, asbestos, or barium oxide. The inorganic nucleating agents are less costly, chemically stable, applicable at high temperatures, and of small size to minimize grain radius. The inorganic nucleating agents may improve dimension stability and mechanical strength, but they tend to aggregate at high temperatures due to their large surface area and fewer coordination sites. The described aggregation results in non-uniform nucleation of polymer, thereby deteriorating the properties of PET.